In U.S. patent application Ser. No. 675,366 filed Nov. 27, 1984 and assigned to the assignee of the present application, there is disclosed an improved exercise machine which makes use of electrically controlled brakes as the resistance force producing elements for the exercise. By applying controlled energization to the brakes, the resistance force, or torque, generated by the brakes along an output shaft may be controlled. The exercise machine is especially adapted so that the resistance elements can be moved to different positions and orientations, and different adaptors can be attached to the brake shafts, so that the machine can be easily converted or adapted to exercise any of a number of different major muscle groups of the body. The control system provided allows the user to select the effort level or resistance force provided by the brakes, and further to select separate resistance forces for opposite directions of movement of the brake shaft corresponding to the opposite half cycles of each exercise repetition. A user can thus set different effort levels for muscle extension and contraction, and the apparatus automatically detects reversal of direction of movement at the end of each half cycle and applies the appropriate preselected resistance force. It is believed that this flexibility of setting different effort levels for each half cycle of the exercise repetition allows for optimum exercise of the various muscle groups.
While this system can provide optimum bidirectional exercise, the present invention provides certain improvements which can lead to better performance and greater usefulness of the exercise machine, particularly for certain types of users. One improvement of the present invention is in the area of accuracy of the selected resistance force levels. It has been discovered that under certain circumstances the actual resistance force or torque provided by the brake unit may vary from the preselected value. The amount of variation may not be significant for most users under most circumstances, but in the worst case can amount to a noticeable difference. This is particularly true in the field of physical therapy, where there is a great need for precisely controlled resistance levels. It has been determined that these inaccuracies result from differences in the torque response of the brake to changed input currents or steady-state input currents. Further, it has been determined that the torque response of the brake in a nonsteady-state condition will vary depending on whether there has been an increase or decrease in applied current. These nonsteady state response characteristics, referred to as hysteresis effects herein, can lead to errors in the resistance force of the exercise machine which, although insignificant for most purposes, can be significant under certain conditions for certain types of users. The present invention provides means for compensating for these hysteresis effects, to provide better control of the resistance force and to eliminate or reduce any differences between the selected resistance force and the resistance force actually obtained.
Another feature of the invention is to provide a range of motion indication so the user can concentrate the bidirectional exercise over a predetermined range of motion. According to this feature, the user can program or select upper and lower, or inner and outer, "limits" for the range of motion for a given exercise. The limits are not hard limits which would actually prevent further movement of the machine and which might cause injury to the user, but preferably are "soft" limits which only provide an indication to the user, such as a tone or an indicator light, when each limit is reached. While generally applicable to all types of exercise, this feature is particularly useful in the field of professional physical therapy, so that the therapist can design a specific exercise for a patient over a prescribed range of motion.